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Variable resistors are often called potentiometers, orpots for short, because one very common use for them isas an adjustable voltage divider. For many years they wereoften called volume controls, because another verycommon use was in adjusting the audio volume produced

by amplifiers, radio and TV receivers.Yet another early name for essentially the same

component when it was used simply as a variable resistancewas rheostat meaning a device to set the flow (ofcurrent).

Pots are made in a variety of physical forms, and with theactual resistance element made from different materials.Some pots are made for frequent manual adjustment via acontrol knob, while others are designed to be adjusted onlyoccasionally with a screwdriver or similar tool, for finetuning of circuit performance. The latter type are usuallycalled preset pots or trimpots.

Most commonly encountered pots designed for manualcontrol have a resistance element consisting of either acarbon-loaded resistance compound ink coated on a fibre

sheet, or a winding of resistance wire (nichrome, copper-nickel alloy or similar). Not surprisingly these are calledcarbon composition and wirewound pots respectively.

Higher quality pots for use in professional equipment mayhave the resistance element made from hot-mouldedcarbon loaded material, or from a metal-and-glass powder(cermet) fired on a ceramic substrate, or a nichrome filmdeposited on a ceramic substrate and then etched in ageometric pattern (bulk metal). These give somewhatgreater reliability, and often better resolution, noise andhigh frequency performance.

Most pots made for manual control are in one of twophysical forms: the rotary type or the linear slider type.As these terms suggest, the sliding tap contact or wipermoves along the resistance element in a circular fashion in

the first type, but in a linear or straight line fashion in thelatter. In a rotary pot the wiper is therefore attached to arotating spindle, while in a slider pot its directly behind thecontrol knob.

Most rotary pots made for manual control offer a totalrotation of about 270 degrees 3/4 of a single turn.However such a limited physical range can make accuratesetting difficult, so multi-revolution pots are also made.These have the resistance element arranged in a spiral orhelix, and the wiper moves along as the control spindle isturned through multiple revolutions (typically 10 or 20).This generally allows much more convenient setting, butsuch pots are not as common as the single-turn typebecause theyre rather more expensive.

Both rotary and slider pots can have a resistance elementof either the carbon composition or wirewound type.Carbon composition types tend to be cheaper to make andfairly quiet in operation, and are also capable of goodresolution. Wirewound types are generally more reliable

and more stable in their performance, but are coarser inresolution due to the way the wiper makes contact withdiscrete turns of the wire element. Wirewound pots canalso be made to dissipate more power, so pots made foruse in circuits handling higher power are generally of thewirewound type.

Trimpots can be made in circular, multiturn circular,linear-slider and multiturn linear-slider form. Low costvarieties generally use an open construction where theresistance element and slider are fully exposed, but as this

allows contamination by dust and moisture, the betterquality trimpots are generally housed inside a small plasticcase which is sometimes hermetically sealed. Somemultiturn trimpots use a worm drive system with a circularelement, while others use a linear element with the sliderdriven via a leadscrew. Either way, the reduction drive isdesigned to have very low backlash to allow smoothand accurate adjustment.

Resistance taperIts very common to see pots described as having either a

linear or log taper. Whats this all about?

Well, the first pots had a linear relationship betweenphysical setting (i.e., rotation) and tapping resistance ratio;for example when the wiper was halfway along the element,

the output voltage was 50% of the total across the wholeelement.

But when these pots were used as audio volumecontrols, it was soon discovered that their adjustmentcharacteristic or law wasnt right. The volume tended togrow far too rapidly as the pot was turned up from zero,while the rest of the pots rotation didnt seem to havemuch control it was often merely adjusting degrees oftoo loud. Just about all of the pots useful range as avolume control seemed to be squeezed into the first 60degrees or so of rotation, making it hard to set the rightlevel.

Engineers soon realised that this problem was due to thehuman ears logarithmicresponse to sound levels. So theremedy was fairly clear: come up with a type of pot that

Electus Distribution Reference Data Sheet: POTPRIMR.PDF (1)

Fig.1: Early true logarithmic taper pots actually hada tapered element...

Fig.2: Todays log taper pots have elements punchedfrom strips with dual-resistivity coatings, whichgivesan acceptable approximation.

VARIABLE RESISTORS OR POTS

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allowed the output level to be adjusted in alogarithmic fashion, rather than a linear one.

The way they first achieved this was to taperthecross-sectional area of the resistance element alongits length, rather than keeping it constant. This gavea low resistance change at first, gradually increasingas the slider moved around or along. By making thetaper a logarithmic one, the pots adjustment lawcould be made logarithmic too. So thats how thelog taper pot came about. The basic idea of a log-taper wirewound slider pot is shown in Fig.1.

Pots with a standard linear adjustment law are stillnecessary for a lot of applications, of course, andthats why theyre available too. Theyre sometimescalled linear taper pots, although that term is

obviously a bit contradictory.Now surprising though it may seem, modern log taper

pots generally dont have a true logarithmic adjustment law.Thats because the tapered element way of making volumecontrol pots was very costly, so engineers soon worked outa much simpler and cheaper way to achieve a roughapproximationof a logarithmic law. This was by making acarbon composition pot with the material used in theresistance element itself varying in composition, so theresistance at the start of rotation is low, and increases laterto achieve a reasonable approximation of a taper.

In fact they soon discovered that quite acceptable volumecontrol could be achieved using a pot with very close to atwo-slope resistance element, with the transition at about50% of rotation. This had the big advantage that the

resistance elements of either rotary or linear-slider potscould be simply punched out of a strip of material coatedwith the two different compositions low and highresistance. (See Fig.2 at lower left.)

Needless to say this approach lowered the cost of makinglog taper pots considerably, so thats why the vast majorityof these pots actually have this kind of roughapproximation logarithmic law.

The resistance/rotation curves for linear, truelogarithmic and log taper pots are shown in Fig.3. Alsoshown are the curves for rotary reverse logarithmic andantilog taper pots, which are virtually the same as truelogarithmic and log taper pots but made for the oppositeor anticlockwiseoperation. These are not commonnowadays, but are still made for a few specialised

applications.Note that when pots with an accurate and truly

logarithmic curve are needed, theyre often still made usinga wirewound element on a tapered former. Either that or abulk metal element with a carefully graduated zig-zagpattern.

Naturally the main use for common log taper pots is forcontrols which need to adjust a quantity in an approximatelogarithmic fashion like audio volume controls. Linearpots are used for most other applications.

ResolutionThe term resolutionis used to refer to the smallest

change in tapping ratio that can be made by moving thepots wiper. Obviously its an important point when a pot

or trimpot is being to accurately adjust circuit operation.The resolution of wirewound pots tends to be fairly

poor, because the element is wound from discrete turns ofresistance wire and the wiper contact can usually only slidefrom one turn to the next. The output of the pot thereforetends to vary in small regular steps, each corresponding tothe voltage drop in one turn of the element.

Pots using an etched bulk metal element tend to have asimilar problem. However pots which use a carboncomposition, hot moulded carbon or cermet element tendto have somewhat better resolution, because the resistanceof their element is more finely graduated.

Needless to say another important aspect of resolution isthe ease of actually moving the pots wiper through a smalldistance, in order to make an adjustment. If you cant dothat easily, the pots electrical resolution tends to beacademic.

So where resolution is really important, multiturn potstend to be more often used than the single-turn variety.

Contact resistanceAnother factor which has an effect on resolution is the

contact resistancebetween the pots wiper and resistanceelement. This also effects the noise which may begenerated by the pot, both when its being adjusted andwhen its simply operating with a fixed setting.

Pots are made with a variety of wiper contact styles andmaterials. Many common carbon composition pots use asimple wiper stamped from nickel-plated spring steel, but

with multiple fingers to give parallel wiping contacts and hence reduce contact resistance.

Higher power wirewound pots may use a wiper with abrush made from a single small block of carbon, but theblock is made with a loading of copper powder to keep thecontact resistance as low as possible. It almost becomes acopper contact with graphite lubrication.

High grade pots with a cermet or bulk metal elementgenerally have a multi-finger spring metal wiper of goldplated phosphor bronze or steel.

Many small trimpots of the cheaper variety have a simplespring metal wiper with a pressed dimple contact whichtouches the element. This can be quite satisfactory whenthe trimpot doesnt get a lot of adjustment, but withfrequent adjustment the contact resistance tends to rise.

Electus Distribution Reference Data Sheet: POTPRIMR.PDF (2)

Fig.3: The rotation vs . resistance ratio law curves forlinear, true logarithmic and log taper pots. Reverse logand antilog taper curves are also shown.

Single and dual-gang rotary pots ofthe compact typehaving a body16mm in diameter.

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Ganged potsAs youre probably aware, multiple pots can be

combined so theyre activated by the same controlspindle. These are known as ganged pots. Generallyonly two pots are combined in this way, but its possibleto have more.

(By the way its also possible to have two potscombined physically but so theyre still adjustedseparately, by means of concentric spindles one

inside the other. These are called concentric pots, andare made mainly for OEM applications.)

In most cases, the elements and their wipers ofganged pots are electrically quite separate, and broughtout to their own terminals. Theyre simply designed tobe adjusted in parallel, via a single control knob.

Generally ganged pots have identical total resistancevalues too, although its of course possible to makethem with quite different values if thats needed. Thiswould normally require a special order from themanufacturer.

For many applications of ganged pots, its importantthat the individual wipers closely follow each other interms of their tapping ratio (for their respectiveelements). In other words, their tracking errorshouldbe as low as possible. This is largely a matter of qualitycontrol during manufacturing, to minimise mechanicaltolerances.

Hop-on and hop-offYoull find the terms hop-on resistance and hop-off

resistance sometimes used in connection with pots, andyou can probably almost guess what they refer to.

Most pots, whether theyre of the rotary or linear-slidertype, and regardless of the type of resistance element theyuse, tend to have metal contact strips at the ends of thefixed element. And the wiper contact generally touches andrests on these metal strips, when its at either extreme ofits travel. However when its moved away from theextreme, it soon hops on to the actual resistance element,and then ultimately hops off at the other end.

Ideally the change of resistance ratio that occurs at thesehop-on and hop-off points should be zero, so that thereare no sudden changes when the pot is used as a volumecontrol, for example. However it isnt easy for potmanufacturers to achieve this; the best they can generallydo is keep the hop-on and hop-off resistances below about1% of the total value of the resistance element. This is solow that it usually cant be detected in most audio andsimilar applications.

High grade pots with hot-moulded carbon, cermet orwirewound elements generally offer very low hop-on andhop-off resistance values.

Pot marking codesAs well as being marked with a character string indicating

the total value of the resistance element for example100K or 1M the case of a pot generally also carries acode letter showing its resistance taper or curve.

Nowadays most pots are marked according to thesimplified taper coding system adopted by Asiancomponent manufacturers:

A = log taperB = linear taper

However in older equipment you may come across potsmarked according to an earlier taper coding system. Notethe potentially confusing differences:

A = linear taperC = log or audio taperF = antilog taper(Copyright Electus Distribution, 2001)

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Fig.4: Key dimensions of the 16mm and 24mm rotarypot sizes stocked by Electus. Both single and dualgang versions are shown.